Since MediaWiki 1.28, CirrusSearch allowes to search for files with a give Type of file with the keyword filetype: (→ T145560). Audio files in Ogg Opus files are classified as multimedia instead of audio as one would expect.
There are 3 Codecs we allow in OGG-Container on Commons: Vorbis (Audio), Theora (Video) & Opus (Audio, successor of Vorbis). If a file contains a Theora-Stream (and maybe an audio-stream) it is correctly classified as 'video', if a file contains only a Vorbis-stream it is correctly classified as audio but if a file only contains a Opus-Stream it is classified as Multimedia.
No other type of files allowed on Commons are classified as multimedia so far.
Searching for "filetype:audio" thus does not result in all Audio-files but the ones in Opus-coding missing. Searching for "filetype:multimedia" only results in Ogg Opus-Files which is also not what one would expect. Due to T151347 there is no work-around by combining both types with the OR-Keyword. An concrete example would be searching on commons: “O du fröhliche filetype:audio” and “O du fröhliche filetype:multimedia” – both show one result and those differ! There is no way to find both.